Impellers used for a pump or the like are mostly manufactured by welding parts, into which a sheet metal material is press-formed, with an automatic welding machine. FIG. 5 is a schematic cross-sectional view of a conventional impeller. As shown in FIG. 5, the impeller 100 is composed of a disk-like main plate 110, blades 120 joined to the main plate 110, and a side plate 130 having a suction port.
A sheet metal material is die-cut and press-formed to produce the blades 120 of the impeller 100, and then the blades 120 of the impeller 100 are joined to the main plate 110 by welding. However, in the conventional impeller, as shown in FIG. 5, radially inner end portions 122 of the blades 120 which are joined to the main plate 110 are so sharp that a large load is applied to a die portion to die-cut the radially inner end portions 122, and that the die portion is worn away in a short term.
Further, when the respective parts are joined to each other with an automatic welding machine, as shown in FIG. 6, main plates 110 are piled on one another before blades 120 are joined to the main plates 110. Each one of piled main plates 110 is raised, transferred, and welded by the automatic welding machine. However, in the conventional impeller, since the main plate 110 is of a flat disk, adjacent main plates are adhered and attracted to each other in the case where the main plates are piled on one another as described above. Accordingly, when each one of the main plates 110 is to be raised, an adjacent main plate is also raised and transferred together to cause problems such as error interruption of the welding machine.